Atmospheric bores occur when the leading edge of one air mass
intrudes
into another of different density.
This leading edge of denser air, the bore, forces the less dense air above
it upward (the crest),
but the force of gravity quickly pulls the dense air back down again (the
trough), causing one or more waves
to form ("gravity waves".) With enough moisture present, the rising air at the front of the bore-induced
wave or waves produces a line or arc of clouds. The cloud line can be broken or continuous
and the clouds within the line
can be rough or smooth in appearance.

Among the most common causes of bores are cold fronts, surface troughs
running out ahead of fronts, or outflow boundaries of rain-cooled air moving
out ahead of a thunderstorm or thunderstorm cluster. Although these
conditions
produce surface to low level phenomena,
bores can be present in any layer of the atmosphere up to jet stream level.
They
often stretch hundreds of kilometers in length but are usually quite shallow in depth, averaging 1-2 kilometers. Their intensity is
mainly dependent upon the speed of the invading air current (gravity
current) and the stability of the invaded air layer (gravity wave
medium.) When the speed of the gravity current/bore is just slightly faster
than the current it's invading, the waves produced are smooth and
undular in appearance
and the bore is called an
undular bore. As the speed of the bore gets faster the waves become
rough and the bore is called turbulent. Whether undular or
turbulent, atmospheric bores often move far out
ahead of the feature that initiated it, and if they move into unstable air,
their lifting
force can produce further convection which in turn may lead to thunderstorms and severe weather.

In this report I'll analyze an atmospheric bore that formed in central Oklahoma and moved eastward on
the morning of April 30,
2010. I'll describe the synoptic and mesoscale conditions at four specific
locations that this wave passed through and show that it satisfied the conditions
of an atmospheric bore. I'll also examine if
it was undular or turbulent.

Before it
dissipated it
showed the reasons why meteorologists watch them closely. All atmospheric
bores,
undular or otherwise, can trigger severe weather, as this one
eventually did in central and northeastern Mississippi (Figure 1.)

The
12Z surface chart in Figure 2a below shows a cold front stretching from Iowa in
the north down into central Oklahoma, where a weak low and another front
trailed southward into Texas. Figures 2a through 2d show that the low
pressure and the section of the cold front in western OK initially moved
quickly from western to central OK (Figure 2a and 2b) but then made little
progress
from 15Z through 21Z, while the section of the front to the north of there
continued on more quickly. In cases like this we often see an initial
"sideswipe" push to the southeast that dies out, but it is enough to send a
wave of energy that continues onward independent of the initial pushing
mechanism, the cold front. This wave of energy (bore) continued onward at a
faster speed than the southern end of the front which had slowed down. In order to see this
more clearly we can turn to the satellite and radar images.

The visible satellite and radar images below in Figures 3 were chosen to
correspond closely with the surface images above. I have also annotated the position
of the bore in the radar images (black line.) In the
satellite image in Figure 3a we can
first find cloud evidence of the bore in eastern Oklahoma at 13Z. Note the
thin line of clear skies from southern Missouri and eastern Oklahoma into
northeastern Texas. This line of clearing was due to subsiding air at the
back edge of an atmospheric bore triggered by the cold front. In the Figure
3b satellite image we can see the bore denoted by the bright white line of
clouds bisecting Arkansas from northeast to southwest. In
Figure 4b the back edge of the bore is also easily
visible. In the Figure 3c satellite image it is even easier to locate the
bore, where an arc of bright white clouds stretches from northwestern
Mississippi to northwestern Louisiana. At around the time of Figures 3d and
4d (23Z) the bore had gradually stopped moving due to a substantial decrease
in push from the cold front and blocking from the
large high to its east (Figure 2d.) Although a little hard to see due to the
waning daylight, in Figure 3d we can see thunderstorms began erupting over
central MS.

Quite important to note in these images (especially
visible in the radar) is how
the distance between the bore and lagging cold front increases substantially
over a few hours in time. Beside making it clear that the bore moved
eastward much faster than the initiating cold front, it makes it much easier
to separate the effects of the bore as opposed to those of the cold front.
Note how the overcast and low clouds
have been greatly reduced by the bore
over all
of Arkansas by 18Z (Figure 3c) and northwestern Mississippi by 23Z (Figure
3d), leaving only broken clouds. So the bore, by forcing the moist air forward
but not very far upward, cleared out the low level stratus and allowed incoming solar
radiation to heat and destabilize the lower atmosphere over Arkansas and
Mississippi. This eventually allowed strong convection to develop.

Now we'll look at the data
for the same four locations noted in the satellite images from Figure 3
above. We'll also look at the
bore strength, which is the height of the inversion after being lifted
(h1) divided by the inversion height before it got lifted (h0). This
value must be between 1 and 2 for an undular bore. Values higher than
2 indicate a non-undular, or turbulent bore.

NOTE: All of the skewT diagrams used
here, although state-of-the-art interpolations from NASA Larc, are
approximations from RUC data rather than from real soundings.

Surface
Indicators for Atmospheric Bores

1. Wind shift, almost always a
veering, and usually from southeast to southwest, however
slight. With undular bores we often see a more noticeable
increase in wind speed and change in direction.

2. Temperature remains steady or
increases. NO temperature decrease.

3. No change in dewpoint is a
positive indicator. An increase or a decrease is neutral.

Upper Level Indicators for Atmospheric
Bores

1. Increase in height of the stable layer (the
inversion was lifted by the bore.)

2. Wind maximum in a thin layer above the surface.
(Winds above and below this layer will be less.)

3. Wind max is always below the top of the
inversion during and after the lifting.

MUSKOGEE, OK (KMKO)

Muskogee, OK is the first location where the cloud and radar evidence
indicated a bore passage. All of the signs we look for were there: Figure 5a shows
a pressure rise at 09Z and a brief
wind shift from southeast to south at 11-12Z with no appreciable temperature or dewpoint change. Figure 5b shows the
inversion was lifted to 775mb and the wind maximum of 31m/s (nose of the bore)
occurred at about 850mb, below the top of the inversion. From 06 to 09Z the
inversion was lifted by the bore from 1630 meters
to 1880 meters.
1880/1630 = 1.15. While this indicates an undular bore it indicates a
very weak one.

Figure 5a.
Meteogram for KMKO, Muskogee, OK to 2010-05-01 06Z. Pressure rise
at 09Z indicates approximate time of bore passage. Note also a
slight wind shift from the SE to S at 10Z. Courtesy
University of
Wyoming.

Figure 5b.
SkewT KMKO 2010-04-30 09Z, same time as circle in Figure
5a. Note the southwest wind max of 31m/s at 850mb and the inversion layer elevated above
it to about 775mb. (see
the lift from 06-09Z.) Courtesy
NASA Larc RUC plot.

FORT
SMITH, AR (KFSM)

At
location KFSM we have more evidence of the bore passage. Figure 6a
shows a pressure rise at 11Z and
a wind shift from south to southwest at 12Z with no appreciable temperature or dewpoint
change. Figure 6b shows the inversion was lifted to 700mb and the wind
maximum of 26m/s occurred at 800-850mb, below the top of the inversion.
From 09 to 12Z the inversion was lifted due to passage of the bore from 1900 meters to 2430 meters.
2430/1900 = 1.27. This also indicates an undular bore.

Figure 6a.
Meteogram for KFSM, Fort Smith, AR to 2010-05-01 06Z. Pressure
rise at 11Z indicates approximate time of bore passage.
Note also a slight wind shift from S
to SW
at 12Z.
Courtesy
University of
Wyoming.

Little Rock, AR also shows solid
evidence of a bore passage. Figure
7a shows
a pressure rise at around 13Z with a
wind shift from southeast to south-southwest at 14-15Z and no appreciable temperature or dewpoint change.
Figure 7b shows the inversion was lifted to roughly 720mb and the wind
maximum of 27m/s occurred just below it about 750mb.
From 09 to 15Z the inversion was lifted by the bore passage from 1930 meters to 2470 meters.
2470/1930 = 1.28, indicating an undular bore.

Figure 7a. Meteogram for KLIT, Little Rock, AR to 2010-05-01
06Z. Pressure rise around 13Z indicates approximate time of bore
passage. Note also a slight wind shift from SE to SSW at 15Z.
Courtesy
University of
Wyoming.

The above meteograms show a distinct pressure rise and
a slight veering of the wind without a notable decrease in temperature or humidity
at the first three locations.
This is a classic surface
signature of a bore. From the skewT diagrams we can see
that the inversion was lifted, and there is a wind maximum from the same direction over a relatively thin layer of the
atmosphere below the top of the layer that has been lifted. This is a classic upper air
signature of a bore. Finally, from the calculations shown, the bore strength
numbers, albeit weak, indicated an undular bore
at all of these locations.

TUPELO, MS (KTUP)

Looking at this last location, Tupelo, MS, in northeastern Mississippi, it appears that as the bore came to a halt
and weakened it still helped
enhance thunderstorm development there. We need to look at the
meteogram in Figure 8 at right and the sequence of skewT images below
in Figure 9 to understand what happened. In the meteogram there are
almost no surface clues of a bore arrival - there are some
minor wind shifts at 21Z but there is no pressure rise that we would
normally associate with a bore's presence at the surface.

The bore
had now slowed to an almost complete stop due to a loss of push from the
now far lagging cold front and resistance from a strong
high pressure cell to its east. Looking at the sequence of skewTs in
Figure 9 below,
we can see the inversion was lifted, presumably by the bore, between 15 and 17Z.
Note also how immediately after this the lapse rates above the cap increased from 17Z to 01Z, when
thunderstorms commenced; especially noticeable from 19Z to 01Z there is a
rapid increase in the moisture content above 900m. The result was a quickly saturated air mass
above the cap, with lapse rates that were clearly unstable. The end result was an outbreak of elevated thunderstorms in northeastern
Mississippi.

For comparison, look at the
development of the
skewT diagrams for Greenville, MS, in the east central part of the
state. Note how the inversion was lifted but the mid-levels remained
very dry. Although both areas initially had a setup for hail and
high winds due to dry air at mid-levels, the atmosphere at Tupelo
eventually became so saturated that it didn't happen there.
But there were areas between the Greenville and Tupelo areas that did
get severe weather with
hail and
high winds.

Figure 8.
Meteogram for KTUP, Tupelo, MS to 2010-05-01 06Z. There is no
concrete surface evidence that the bore arrived or passed this
location. However it is clear that thunderstorms developed just
after 00Z. Courtesy
University of
Wyoming.

Figure 9. SkewT diagrams for KTUP, Tupelo, MS from 2010-04-30 15Z
through 2010-05-01 at 02Z. The inversion is still evident in
these analyses, and after an initial lift from the bore it slowly
weakened. What is more
evident is that the air parcels above the bore have gradually become saturated.
Elevated thunderstorms were occurring at Tupelo by 01Z.
Larger version.

Conclusion

The
evidence indicates that this was a significant atmospheric bore, and
it was weakly undular. Though the cloud evidence was not as impressive
as in other undular bores I've studied, there was certainly visual evidence of a single leading wave at 13Z,
followed by a set of waves at 15Z which were slightly undular in
appearance. This fits with the low bore strength values of
1.15 to 1.28, i.e. it was a weakly undular bore.

As
is common with many atmospheric bores, this one was initiated by a fairly strong,
uniform southeastward push from a cold front. But even as the cold front slowed
down considerably, the bore, because of its initial momentum, continued forward at a faster clip
than the feature that initiated it. Due to its momentum, it continued on at
a fairly fast clip, not slowing down until it entered
eastern and central Mississippi, where in addition to losing momentum it
experienced blocking to the east. For most of its journey this
particular bore pushed the stable layer of very moist air ahead of it
forward, but not very far upward, because the air
layer it moved through was a stable inversion, and by definition bores duct
their energy forward through a stable layer with very little of its
energy getting lost in upward motion. (It is the layer of conditionally
unstable layer above the inversion that gets lifted into a thin cloud line
above it that shows us the bore's cloud signature.) But as the
bore slowed down to a stop over central Mississippi, it appears that, due to
blocking to the east, it no longer could lift the stable layer, and its
remaining momentum was translated upward. The evidence bears this out: it
appears the humid air it contained was forced upward above the stable
inversion that existed below 900-950mb, causing numerous thunderstorms over
northeastern Mississippi, some of which were
severe.

Although an indirect cause, but in some ways more
significant, the bore appears to have cleared out
the lower level stratus and fog over Arkansas, allowing considerable
incoming solar radiation and a large increase in CAPE. The resulting
thunderstorms triggered by the ensuing cold front were catastrophic,
resulting in deadly, tornadic thunderstorms that resulted in considerable
deaths and damage over Arkansas.